Return to the selection tool used in the first step but this time use the Mode selection box choosing fixed Proportions and type in that you want the width to be 2 and the height to be 1. Draw a selection box by clicking where you want the upper left corner and (holding down the mouse button) drag to the lower right. Be sure to start and finish at the edges if you want the result to be the full 1200 pixels wide and 600 pixels high. The box position can be adjusted after it is drawn by using the up and down arrow keys. Pull down the Image menu and click on crop to eliminate everything outside the box.